Multiprotocol Label Switching (MPLS) enables efficient delivery of a wide variety of differentiated, end-to-end services. Multiprotocol Label Switching (MPLS) traffic engineering (TE) provides a mechanism for selecting efficient paths across an MPLS network based on bandwidth considerations and administrative rules. Each label switching router maintains a TE link state database with a current network topology. Once a path is computed, TE is used to maintain a forwarding state along that path.
As described in more detail in various Internet Engineering Task Force (IETF) Request for Comment (RFC), such as RFC4726 and RFC5151, an Area Border Router (ABR) is a router located between several areas in a hierarchical Open Shortest Path First (OSPF) network. ABRs maintain topology information from multiple areas. In the case of Resource Reservation Protocol (RSVP) Inter-Domain TE-LSPs of type Contiguous LSP each Area Border Router (ABR) triggers a path computation (also referred to as an ERO expansion), before forwarding the RSVP Path message downstream. Thus, each ABR is responsible for calculating TE constrained path for its successive TE-Domain(s) or Area(s). Every such ABR that triggers path a computation for its TE-Domain can have multiple equal-cost paths and has to choose one of them.
In the case of a LSP setup spanning multiple TE domains or areas, it is possible for an ingress Label Edge Routers (LER) or subsequent ABR node doing an ERO expansion to have available to it multiple un-equal cost paths to reach a next ABR or a destination of the LSP. If a given node fails ERO expansion and the node happens to be associated with the best path to reach the destination, the LSP setup may fail until the network re-converges, which may not happen for an extended period of time. Further, this LSP setup failure may occur even though an alternate path exists since the LSP setup process may never use the alternate path if it is deemed to be non-optimal.